Process and facility for removing metal contaminants from fertilizer streams

Abstract

A process and system for producing fertilizers with reduced levels of contaminants, and particularly a flue gas scrubbing process and facility that produce a fertilizer byproduct which is treated to remove heavy metals originally present in the scrubbed flue gas. The process entails contacting a flue gas with a scrubbing solution to remove acidic gases and produce a byproduct containing a fertilizer compound precipitate and dissolved metals and/or metal compounds. A fertilizer solution containing dissolved fertilizer and dissolved metals and/or metal compounds is then produced by adding water to the byproduct, after which the pH of the fertilizer solution is adjusted one or more times with a gaseous and/or liquid alkali to form precipitates of one or more metal species. The precipitates are then removed from the fertilizer solution and the solution dewatered to yield a fertilizer product with lower contaminant levels.

What is claimed is:  
     
       1. A flue gas scrubbing process in which a flue gas containing acidic gases and metals and/or metal compounds is contacted with an ammonium sulfate solution to absorb sulfur dioxide and produce a byproduct containing ammonium sulfate precipitate and dissolved metals and/or metal compounds, the method comprising the steps of: 
       producing an aqueous fertilizer solution containing dissolved ammonium sulfate and dissolved metals and/or metal compounds by adding water to the byproduct;  
       adjusting the pH of the aqueous fertilizer solution with gaseous and/or liquid ammonia to form precipitates of at least one metal specie;  
       removing the precipitates of the metal specie from the aqueous fertilizer solution;  
       adjusting the pH of the aqueous fertilizer solution with gaseous and/or liquid ammonia to form precipitates of at least a second metal specie;  
       removing the precipitates of the second metal specie from the aqueous fertilizer solution; and  
       dewatering at least a portion of the aqueous fertilizer solution to precipitate ammonium sulfate.  
     
     
       2. A method according to  claim 1 , further comprising at least a third adjusting step performed before the dewatering step, in which the pH of the aqueous fertilizer solution is decreased with sulfuric acid to form precipitates of at least a third metal specie, after which the precipitates of the third metal specie are removed from the aqueous fertilizer solution. 
     
     
       3. A method according to  claim 1 , wherein the flue gas is produced by a fossil fuel combustion apparatus, and the flue gas is delivered from the fossil fuel combustion apparatus to a gas-liquid contactor where the flue gas is contacted with the ammonium sulfate solution. 
     
     
       4. A method according to  claim 1 , wherein the gaseous and/or liquid ammonia is recovered from the aqueous fertilizer solution prior to the dewatering step. 
     
     
       5. A method according to  claim 4 , wherein the gaseous and/or liquid ammonia recovered from the aqueous fertilizer solution is used in the adjusting step to adjust the pH of the aqueous fertilizer solution. 
     
     
       6. A method according to  claim 4 , wherein the gaseous and/or liquid ammonia recovered from the aqueous fertilizer solution is added to the ammonium sulfate solution for absorbing additional sulfur dioxide from the flue gas. 
     
     
       7. A method according to  claim 1 , wherein after the removing step at least a portion of the aqueous fertilizer solution is added to the ammonium sulfate solution for absorbing additional sulfur dioxide from the flue gas. 
     
     
       8. A flue gas scrubbing facility that produces as a byproduct a fertilizer with reduced levels of contaminants, the flue gas scrubbing facility comprising: 
       means for contacting a flue gas with a scrubbing solution to remove acidic gases from the flue gas and produce a byproduct containing a fertilizer compound precipitate and dissolved metals and/or metal compounds;  
       means for adding water to the byproduct in an amount sufficient to produce an aqueous fertilizer solution containing dissolved fertilizer and dissolved metals and/or metal compounds;  
       means for adjusting the pH of the aqueous fertilizer solution with a gaseous and/or liquid alkali to form precipitates of at least one metal specie; and  
       means for removing the precipitates of the metal specie from the aqueous fertilizer solution.  
     
     
       9. A flue gas scrubbing facility according to  claim 8 , wherein the adjusting means adds gaseous and/or liquid ammonia to the aqueous fertilizer solution to increase the pH thereof. 
     
     
       10. A flue gas scrubbing facility according to  claim 8 , wherein the adjusting means adds sulfuric acid or hydrochloric acid to the aqueous fertilizer solution to decrease the pH thereof. 
     
     
       11. A flue gas scrubbing facility according to  claim 8 , wherein the adjusting means is operable to adjust the pH of the aqueous fertilizer solution multiple times to form precipitates of multiple metal species. 
     
     
       12. A flue gas scrubbing facility according to  claim 8 , wherein the aqueous fertilizer solution comprises at least one of ammonium sulfate, ammonium nitrate, potassium sulfate, potassium nitrate, and potassium chloride. 
     
     
       13. A flue gas scrubbing facility according to  claim 12 , wherein the adjusting means adds gaseous and/or liquid ammonia to the aqueous fertilizer solution to increase the pH thereof. 
     
     
       14. A flue gas scrubbing facility according to  claim 12 , further comprising means for recovering the gaseous and/or liquid alkali from the aqueous fertilizer solution following the removing step. 
     
     
       15. A flue gas scrubbing facility according to  claim 14 , further comprising means for dewatering the aqueous fertilizer solution and reprecipitating the fertilizer compound precipitate after recovering the gaseous and/or liquid alkali from the aqueous fertilizer solution. 
     
     
       16. A flue gas scrubbing facility according to  claim 8 , wherein the scrubbing solution contains ammonium sulfate. 
     
     
       17. A flue gas scrubbing facility that uses an ammonium sulfate solution to absorb sulfur dioxide from a flue gas that contains metals and/or metal compounds, and which produces a byproduct containing ammonium sulfate precipitate and dissolved metals and/or metal compounds, the flue gas scrubbing facility comprising: 
       means for producing an aqueous fertilizer solution containing dissolved ammonium sulfate and dissolved metals and/or metal compounds by adding water to the byproduct;  
       means for adjusting the pH of the aqueous fertilizer solution with gaseous and/or liquid ammonia to form precipitates of multiple metal species;  
       means for removing the precipitates of the multiple metal species from the aqueous fertilizer solution; and  
       means for dewatering at least a portion of the aqueous fertilizer solution to precipitate ammonium sulfate.  
     
     
       18. A flue gas scrubbing facility according to  claim 17 , further comprising a fossil fuel combustion apparatus in which the flue gas are produced, and a gas-liquid contactor to which the flue gas is delivered from the fossil fuel combustion apparatus and where the flue gas is contacted with the ammonium sulfate solution. 
     
     
       19. A flue gas scrubbing facility according to  claim 18 , further comprising means for recovering the gaseous and/or liquid ammonia from the aqueous fertilizer solution prior to the aqueous fertilizer solution being delivered to the dewatering means. 
     
     
       20. A flue gas scrubbing facility according to  claim 19 , further comprising means for delivering the gaseous and/or liquid ammonia recovered from the aqueous fertilizer solution to the adjusting means. 
     
     
       21. A flue gas scrubbing facility according to  claim 19 , further comprising means for delivering the gaseous and/or liquid ammonia recovered from the aqueous fertilizer solution to the gas-liquid contactor. 
     
     
       22. A flue gas scrubbing facility according to  claim 19 , further comprising means for delivering at least a portion of the aqueous fertilizer solution to the gas-liquid contactor.